stbt.Display.__init__: (save-video) Remove unnecessary argv[0] check

The only callers of `stbt.Display.__init__` are:

    stbt.Display.__init__
    ↳ stbt.init_run
      ↳ stbt-run: stbt.init_run(..., args.save_video)
      ↳ stbt-record: stbt.init_run(..., False)

`stbt-record` calls `init_run` with `save_video=False`, so it is
redundant to check that `argv[0] == "stbt-run"`.

Thanks to Bryce Gibson for pointing this out:
https://groups.google.com/d/msg/stb-tester/jEBSeS9-l2c/LXX7J2DAcpgJ